Conventional rail locomotive designs typically employ a plurality of traction motors for propelling and retarding the forward and reverse motion of the locomotive. When being used to propel the locomotive the traction motors function as electric motors which convert electrical current into kinetic or mechanical energy. For example, current locomotives typically include a diesel engine which is used to drive an alternator which generates and supplies an electrical current to a plurality of traction motors which, in turn, converts this electrical energy into mechanical energy causing the locomotive to be propelled in the forward or reverse direction.
However, the traction motors may also be configured to function as generators to produce a dynamic braking action which is used to slow the movement of the locomotive by converting the kinetic energy of the locomotive into electrical energy. Unfortunately however, this electrical energy cannot be used or stored conveniently on-board the locomotive. As such, this energy must be dissipated. To accomplish this the traction motors are connected to a bank of electrical resistors called a dynamic braking grid and the electrical energy generated during the dynamic braking action is converted into thermal energy, or heat, using resistive elements within the dynamic braking grid wherein the dynamic braking grid is typically force cooled by fan-driven airflow which transfers the heat energy into the ambient environment.
However, certain design and physical characteristics of current dynamic braking grid resistor packages, such as the package size and the upper temperature limit of the materials used to construct the dynamic braking grid tend to limit the amount of dynamic braking power that may be applied to the locomotive and still be efficiently transferred into the ambient environment. This acts to limit the amount of power that may be dissipated by a grid at a given ambient condition based on temperature and pressure. For example, a typical stack of braking grids occupying a volume of approximately 50 cubic feet may only be able to dissipate 1.8 MW of power. As such, because the efficient transfer of heat energy from the resistors to the ambient environment is a critical factor to the proper performance of a dynamic braking system, it is desirable to maximize this efficiency. Unfortunately however, because current dynamic braking grid resistor package designs are subject to cost, size, weight and noise limitations and the amount of space available on board the locomotive, it is not practical to simply increase the size of the grid enclosure or the size, quantity and/or the capacity of the cooling fans and resistors.
One way that has been developed to help solve this problem involves maximizing the energy dissipated across the entire grid by minimizing “hot spots” in the braking grid while avoiding localized material failure. Referring to FIG. 1, FIG. 2 and FIG. 3, typical cooling fans provide an uneven airflow velocity distribution at the outlet of the fan, wherein the outlet airflow velocity is highest proximate the center of the fan blade and lowest at the root and tips of the fan blades. In order to control the airflow more efficiently, an airflow diffuser plate is disposed between the fan outlet and the grid package inlet. For example, a typical cooling fan 100 used in a dynamic braking grid resistor package is shown and includes an impeller fan blade 102 and a flat plate 104, wherein the flat plate 104 defines a plurality of diffuser holes 106. Flat plate 104 includes an annular ring portion 108, a central portion 110 and a plurality of corner portions 112, wherein the annular ring portion 108 is aligned with the high velocity components of the fan air flow and wherein the central portion 110 and the corner portions 112 are aligned with the low velocity components of the fan air flow. The annular ring portion 108 defines a plurality of holes 114 having a relatively low quantity and/or size and the central portion 110 and the corner portions 112 define a plurality of holes 116 having a relatively high quantity and/or size.
As can be seen in FIG. 3, the uneven distribution of openings in the flat plate 104 has the effect of making the distribution of airflow volume and velocity downstream of the flat plate 104 much more uniform than that provided at the fan outlet. Additionally, the flat plate 104 also serves to reshape the air stream from the generally circular cross-sectional shape of the cooling fan 102 into the generally rectangular cross-sectional shape of the grid package, providing a more evenly distributed airflow within the grid package. This acts to cool the resistor grid in a more evenly distributed manner. Unfortunately however, the flat plate 104 also acts as a flow restriction and causes a significant pressure drop in the air stream which reduces the mass flow rate of cooling air available for flow through the resistor grid package. This is undesirable because the cooling of the resistive elements remains inefficient.
With conventional systems having a fan and six resistors, it is difficult to lower the maximum grid operating temperature. For a standard system, due to successive grid heating, the discharge resistor experiences the highest temperature which limits the power and environmental capability of the system. Thus, the cooling of a conventional system is difficult to improve upon for a number of reasons such as, airflow could be improved via increased flow via increased fan size, but space is limiting, airflow could be improved via increased fan speed, but physical fan/motor stresses are constraining, airflow could be increased but noise levels are constraining, airflow could be increased by reducing the pressure drop across the system, but inlet losses are limited by the size of the system, and grid and diffuser losses are limited by the size of the system as well. For instance, grid pressure drops could be reduced by increasing grid spacing, but the grid would have to be larger to still maintain sufficient heat transfer area. In fact, with conventional systems, blowing snow and freezing condensation can enter the fan, resulting in it freezing.